1. Field of the Invention
The invention relates to microwave resonators generally used in the field of terrestrial or space communications.
2. Description of the Prior Art
A microwave resonator is an electromagnetic circuit that is tuned to pass energy at a precise resonant frequency.
Microwave resonators can be used to produce filters to reject the frequencies of a signal that are outside the pass-band of the filter.
A resonator takes the form of a structure forming a resonant cavity whose dimensions define the required resonant frequency.
Accordingly, any change in the dimensions of the cavity introducing a change in the volume thereof causes a shift in its resonant frequency and consequently a change in the pass-band of the filter.
Changes in the dimensions of a resonant cavity may result from expansion or contraction of the walls of the cavity caused by changes of temperature, and increase in proportion to the coefficient of thermal expansion of the material.
There are several prior art techniques for compensating the variation in cavity volume caused by changes in temperature in order to maintain the resonant frequency at the predefined value under normal temperature conditions (ambient temperature around 20° C.).
These techniques are usually based on the use of components that are part of the structure of the cavity itself and are made of materials with different coefficients of thermal expansion, one of the coefficients being much lower than the other(s). These components are arranged so that expansion occurs in opposite direction, causing a deformation of the cavity if the temperature rises that reduces its volume.
A first material is conventionally used having a very low coefficient of thermal expansion, such as Invar® (Registered Trade Mark). The second material used is generally aluminum, which has a higher coefficient of thermal expansion than Invar® but which on the other hand, in addition to its low density and therefore its lightness, has a high thermal dissipation power, making it particularly suited to space applications.
There also exist compensation devices external to the cavity based on the same principle of using two materials with different coefficients of thermal expansion. For more details see the description of a temperature compensation device given in patent application EP 1 187 247 of 28 Aug. 2001, for example.
The drawback of these various solutions is that each temperature compensation device must have dimensions adapted to the length of the resonant cavity with which it is associated or of which it forms part. Thus temperature compensation devices must be produced with dimensions adapted to each different cavity length.
The present invention solves this problem by proposing a system equally suited to cavities of the same length and to cavities of different lengths.
The invention further proposes a temperature compensation system the adjustment device whereof confers high temperature stability on the resonator.